The present invention relates to an improved dampening device, utilizing a water and water-alcohol mixture, for use on offset printing machines.
Offset printing machines are well known in the art, and accordingly a detailed description thereof is deemed not to be necessary.
However, to facilitate complete understanding of the present invention, it is pointed out that in the offset lithographic system, the "form" or "plate" is completely flat, i.e. is not in relief, and inking thereof is carried out by exploiting a well known physical-chemical process.
This process, which forms the basis of offset photographic inking, causes a repulsion between an oily substance, which in the present case is the printing ink and water, or more generally the dampening liquid.
The surface of the plates, made of zinc, aluminum or of multiple different metals, on which has previously been formed the subject to be printed, has to be moistened before the inking operation in order to assure the necessary repulsion of the ink from the non-engraved or etched portions.
Accordingly, each printing unit of the offset printing machines is provided with a dampening or moistening device which acts to moisten or dampen the plate as wound onto the plate cylinder.
These dampening devices comprise a plurality of counter-rotating rollers, in part driven and in part frictionally-moved by the adjacent rollers, including at least a dipping roller, which is partially immersed in the dampening liquid, and at least a so-called "dampening" roller.
This latter tangentially contacts the plate cylinder and another intermediate roller which may itself receive, from a further roller and by tangential contact, the dampening liquid from the dipping roller.
The dampening roller, or rollers is or are generally coated with rubber and, if necessary in order to assure an optimal supplying of the liquid, with a braiding or textile covering.
Advantageously, one of the intermediate rollers performs a reciprocating axial movement in order to aid a homogenous distribution of the dampening liquid, which is taken up and carried by surface adhesion.
For this reason, and on the analogy of the rollers provided with this type of movement and which are present in the inking assemblies, it is called a "vibrating" roller.
It is also known that in offset printing it is essential to maintain an equilibrium between the water and ink, and that the ideal equilibrium is obtained if it is possible to reduce to a minimum the amount of water supplied to the plate, and to adjust the ink under these conditions of minimum dampening.
The metering of the dampening liquid, transferred to the plate, is obtained in various ways: for example, one of the intermediate rollers, in particular that referred to as the transfer roller, which is located adjacent to the dipping roller, is supported in such a way as to be shiftable parallely to itself, and is cyclically moved or driven in such a way as to periodically moved away from and then again rest on said dipping roller.
Due to this movement, the amount of liquid transferred onto the plate is a function of the ratio of the effective time of operation of the printing unit, and the sum of the intermittent contact times of the transfer and dipping rollers.
It should furthermore be pointed out that additional members are used in order to aid the metering of the dampening liquid, and an even distribution of the liquid film onto the transversal dimension of the system, i.e. in the direction of the roller axes.
These members may carry out a selective distribution of the liquid, with more at the centre or at the sides of the rollers, depending on printing requirements.
In particular, there is used an auxiliary roller, the so called distributing or metering roller, which tangentially contacts, with an adjustable pressure, the first dipping roller of the series of rollers which commences with the dipping roller, and hence with the dampening roller or rollers, and supported to be pivotable in a tangential plane perpendicular to the plane of the axes of said rollers.
Furthermore, in some machines, there is adopted a technical solution according to which the dampening roller or one of the dampening rollers also carries out the function of the end inking roller of the system or of one of the inking systems or assemblies, thereby supplying onto the plate a liquid film formed by a mixture or emulsion of ink and wetting liquid.
The more conventionally and usually used dampening liquid is water, which however presents the drawback of the instability of the water-ink optimal-equilibrium, the obtaining and maintaining of which are indispensable conditions for a quality production.
This equilibrium is more and more difficult to be obtained as, in order to provide a proper tonality of colour, the water amount has to be reduced.
In relatively recent years, has been introduced the use of suitably metered water-alcohol mixtures, which provided the advantages well known to those skilled in the art.
However, the use of alcohol requires the adoption of particular expedients of preparation or setting, and especially skilled operators.
Furthermore, this use may be difficult, or even impossible, under some environmental conditions such as, for example, in tropical climates.
In relatively crecent years it has been attempted to solve this problem by providing machines which, when suitably adapted, may be constructed or set to operate both with the conventional water dampening and with alcohol dampening, i.e. with a water-alcohol mixture.
However, these latter types of machines have until now been affected by considerable drawbacks.